The present invention relates to a manufacturing process for a mirror finished silicon wafer capable of manufacturing the mirror finished silicon wafer having an excellent quality in which grown-in crystal defects are annihilated by heat-treating the mirror finished silicon wafer in a gas atmosphere of high safety at a lower cost without selection of a heat treatment furnace for use in the heat treatment, a mirror finished silicon wafer having an excellent quality, and a heat treatment furnace preferably used in the manufacturing process.
It has been known that there exist defects named so-called grown-in defects such as COP (Crystal Originated Particle), oxide precipitates and so on in a CZ silicon wafer. A proposal has been made on a heat treatment performed in a hydrogen atmosphere (hereinafter may be referred to as xe2x80x9chydrogen annealingxe2x80x9d) as a method for annihilating grown-in detects in the vicinity of a wafer surface. This heat treatment is required to use hydrogen at a temperature of 1000xc2x0 C. or higher, so it is necessary to take a countermeasure from the viewpoint of safety. Since such a treatment cannot be carried out in an ordinary open type furnace (a furnace with an unsealed opening side such as a horizontal furnace), the furnace is required to be modified with a sealed structure for increasing airtightness and an explosion-proof apparatus as a measure against an explosion, which have lead to a very high cost.
In FIG. 3, there is shown a schematic structure of an ordinary horizontal furnace. In FIG. 3, reference numeral 10 indicates a horizontal furnace, which has a quartz tube body, that is, a reaction tube 12. A gas supply port 14 for supplying a gas is provided at a front end of the reaction tube 12. At the rear end of the reaction tube 12 is provided a furnace opening 16 which is capable of opening and shutting by a cap 18. Where a hole 20 is formed in the cap 18, a supply gas is released to the outside of the furnace mainly through the hole 20. Where no hole 20 is formed, the supply gas is released to the outside of the furnace through a clearance between the cap 18 and the furnace opening 16. A wafer support Si boat 22 supporting vertically many wafers W is placed inside of the reaction tube 12. A heater 24 is provided outside of the outer periphery of the reaction tube 12 and the many wafers W placed inside of the reaction tube can be heat-treated.
Meanwhile, it has been recently found that even a heat treatment carried out in an argon atmosphere (hereinafter may be referred to as xe2x80x9cAr annealingxe2x80x9d) can annihilate the grown-in defects in the level equal to hydrogen annealing. Ar annealing is not explosive and then safer compared with hydrogen. Although the Ar annealing ensures safe operation, it has also been known that the annealing displays a characteristic behavior to a silicon wafer.
An example of such a characteristic behavior is a phenomenon that tiny pits are easily formed on a surface of a wafer subjected to the Ar annealing. This is caused by the following mechanism. An oxide film is formed by very small amounts of oxygen and water as impurities included in a raw material gas, or oxygen and water in the outside air involved through the furnace opening of the reaction tube in a heat treatment process, and then the oxide film is allowed to react with silicon (Si) according to the following reaction:
SiO2+Sixe2x86x922SiO
As a result of the reaction, Si is etched and the etched portion is observed as pits. The pits serve as a cause for degrading a local surface roughness (micro-roughness) and a long-period surface roughness (haze) on a wafer surface. Thus, an Ar gas is sensitive to a trace of impurities and small changes in the environment such as fluctuations in temperature, so the Ar gas has a demerit of difficulty in handling.
As measures to prevent this phenomena from occurring, two methods have been mainly proposed: One proposal is that an impurity content in a raw material gas is restricted to 5 ppm or less, and a purge box is also provided at an opening of a heat treatment furnace to prevent the outside air from being involved when wafers are inserted into the furnace (JP A 99-135511).
The other proposal is a method in which wafers are kept at 300xc2x0 C. or lower and inserted into the furnace in order to prevent the outside air from being involved into the furnace when the wafers are inserted into the furnace (JP A 99-168106). However, it is supposed with ease that these methods lead to complexity in the apparatus and lower productivity.
As described above, the hydrogen annealing and the Ar annealing can advantageously annihilate the grown-in defects and give an excellent oxide film dielectric breakdown strength characteristic, so they have been widely used recently. However, for the above-mentioned reasons, it has been considered that high quality annealed wafers cannot be produced in a furnace with poor airtightness such as a horizontal furnace.
In the recent trend that device makers and wafer makers introduce many heat treatment furnaces, an increasing number of the makers have introduced vertical furnaces for the purpose to save a floor space. A vertical furnace has been developed later than a horizontal furnace and enables a highly airtight structure; therefore, the vertical furnace has been profitably used in a variety of applications. Accordingly, in view of such recent introduction of the vertical furnaces, effective use of long-standing horizontal furnaces has been sought. However, since the horizontal furnaces lack airtightness as described above, the range of its use has been still limited.
As stated previously, the presence of the grown-in defects such as COP (Crystal Originated Particle) is taken up as one of causes for decreasing a product yield in a device process. The grown-in defect is one of causes for degrading the oxide film dielectric breakdown strength and disconnecting wiring. Particularly, the defects are the greatest factor for deterioration in the oxide film dielectric breakdown strength. In order to annihilate the COP, it has been found that the hydrogen annealing and the Ar annealing are effective. The hydrogen annealing is, however, problematic in a safety aspect because of the use of the hydrogen gas at high temperature. In order to avoid such a problem, there is used a safety apparatus which leads to complex and expensive facilities for the hydrogen annealing as well as to a decrease in productivity and an increase in a production cost.
On the other hand, the Ar annealing has a problem that pits are easily generated if an Ar gas is of low purity. Furthermore, at the same time that pits are generated, micro-roughness and haze on a wafer surface are deteriorated. It has been known that micro-roughness and haze affect the oxide film dielectric breakdown strength and mobilities of electrons and holes just under an oxide film of a transistor having a MOS structure (see J. Appl. Phys. 79(2), Jan. 15, 1996, p. 911). Especially, mobilities of carriers (electrons and holes) are required to improve with an increase in degree of integration of MOS transistors. It is therefore necessary not only to decrease grown-in defects, but also to reduce micro-roughness and haze.
The inventors have conducted, as shown in Experimental Example 1 described later, an investigation into and a research on a haze level on a wafer surface after Ar annealing performed in a horizontal furnace, which is generally used, of low cost and widely spread, and as a result, have found for the first time that the outside air intrudes into a reaction tube for a heat treatment through a poorly sealed part of a connection portion between the reaction tube and a supply line of a raw material gas, which deteriorates a haze level of a heat-treated wafer.
That is, a connection portion 28 between a gas supply port 14 of a front end of a quartz tube body (reaction tube) 12 of a heat treatment furnace 10 which is generally used as shown in FIG. 3, and a supply line 26 of a non-oxidative raw material gas is, in many cases, connected with a joint 30 made of fluorocarbon resin as shown in FIG. 2. A great amount of the raw material gas flows through the connection portion 28; therefore, it has been considered that there is no chance for the outside air to intrude into the interior of the quartz tube body. As a result of the inventors"" detailed investigation, however, it has been found that if the connection portion 28 has a little leak, when a great amount of a raw material gas flows, the outside air is easy to intrude into the interior according to Bernoulli""s theorem. The present invention has been made on the basis of such findings as obtained in the investigation.
It is an object of the present invention is to provide a manufacturing process for a mirror finished silicon wafer capable of manufacturing a mirror finished silicon wafer having an excellent quality in which grown-in crystal defects are annihilated by heat-treating the silicon mirror finished wafer in a heat treatment in a gas atmosphere of high safety at a lower cost without selection of a heat treatment furnace for use in the heat treatment, a mirror finished silicon wafer having an excellent quality, and a heat treatment furnace preferably used in the manufacturing process.
In order to achieve the above described object, according to the present invention there is provided a manufacturing process for a mirror finished silicon wafer comprising the steps of: connecting a reaction tube of a heat treatment furnace to a supply line for a non-oxidative raw material gas via a connection portion; supplying a non-oxidative gas into the reaction tube through the supply line and the connection portion; and heat-treating the mirror finished silicon wafer In the heat treatment furnace in an atmosphere of a non-oxidative gas, wherein a content of impurities in the non-oxidative gas supplied Into the reaction tube is 3 ppm or less. If an impurity concentration in the non-oxidative gas supplied Into the reaction tube of the heat treatment furnace is 3 ppm or less, preferably 1 ppm or less and more preferably 0.5 ppm or less, even when a wafer is annealed In the non-oxidative gas, a surface state of the wafer is not deteriorated.
As impurities contained in the non-oxidative gas, there can be named impurities originally contained in the non-oxidative raw material gas and/or the outside air intruding into the reaction tube.
In order to prevent the outside air from intruding into the reaction tube from the connection portion, the connection portion is preferably of a flange structure.
It is preferable to prevent the intrusion of the outside air to the possible lowest level from a furnace opening with a supply amount of the non-oxidative raw material gas being 15 l/min or more during the heat treatment.
In consideration of safety, there is used as the non-oxidative raw material gas, an Ar gas or an Ar gas including a hydrogen gas the content of which is equal to or less than a lower explosion limit (about 4% or less).
Advantages as described below can be enjoyed with any of a horizontal furnace and a vertical furnace in the above-mentioned heat treatment.
In cases where a process of the present invention is performed with a horizontal furnace, the advantages are listed as follows: (1) An annealed wafer with a good haze level which has not been obtained with a conventional horizontal furnace can be attained, (2) a heat treatment furnace in use requires no airtightness; therefore, an existing horizontal furnace can be put into practical use by simple and low cost improvement only, by which the application range of the horizontal furnace increases, and (3) there do not occur demerits such as (a) particles and (b) slip dislocations which arise when using a vertical furnace.
(a) As for the particles, the following explanation is given: In a case of a vertical furnace, in order to prevent a mirror finished surface side of a wafer (on which a device is fabricated) from contacting the boat, the wafer is usually placed on the boat with the mirror finished surface side up. Therefore, particles floating in the space within the heat treatment furnace fall by gravity to easily attach to the mirror finished surface sides of the wafer. In contrast to this, since in a horizontal furnace a wafer is heat-treated in a vertical position, there is free from such a fear.
(b) As for the resistance to the slip dislocations, the following explanation is given: In a case of a vertical furnace, a wafer is required to be supported almost horizontally; therefore, a stress acted on a supporting portion of the wafer increases by its own weight and hence longer slip dislocations are easily generated compared with a horizontal furnace. In a case of a horizontal furnace, however, wafers are held almost vertically; therefore, a stress on a wafer caused by its own weight is comparatively small, slip dislocations being hard to occur.
Furthermore, a process of the present invention is performed using a vertical furnace with merits compared with the conventional process in regard to the following points: (1) An annealed wafer with an improved haze level can be obtained compared with a conventional vertical furnace (a vertical furnace having airtightness as in a hydrogen annealing furnace), (2) since airtightness is unnecessary even in a vertical furnace, the number of necessary parts can be reduced compared with a hydrogen annealing furnace which requires airtightness, thereby enabling a low-priced, simple and convenient vertical furnace to be realized, and operability and reliability thereof to be improved.
A mirror finished silicon wafer of the present invention is manufactured by a manufacturing process for a mirror finished silicon wafer of the present invention, wherein a haze level is 0.1 ppm or less on the whole surface of the wafer, and a P-V value is 1.5 nm or less and a Rms value is 0.15 nm or less in each 2 xcexcmxc3x972 xcexcm area thereof. Such a wafer has more excellent surface roughness across the whole surface of the wafer uniformly compared with an ordinary mirror finished wafer, so an extremely good oxide film dielectric breakdown strength characteristic can be obtained on the whole surface of the wafer.
A heat treatment furnace of the present invention is for beat-treating a mirror finished silicon wafer in an atmosphere of non-oxidative gas and comprises: a reaction tube of the heat treatment furnace; a supply line for a non-oxidative raw material gas; and a connection portion connecting the reaction tube and the supply line, wherein the non-oxidative gas is supplied through the supply line and the connection portion, and an intruding amount of the outside air from the connection portion during the heat treatment is 1 ppm or less of a supply amount of the non-oxidative raw material gas. Since a degree of deterioration of a surface roughness of an annealed wafer is affected largely by an intruding amount (a leakage) of the outside air from the connection portion, the intruding amount is necessary to be limited to at least 1 ppm or less and preferably 0.05 ppm or less in order to obtain a surface roughness of the same level as that of an ordinary mirror finished wafer.
It is preferable to prevent the intrusion of the outside air to the possible lowest level with a flange structure of the connection portion.